Summary of Significant Accounting Policies - Remaining Performance Obligations (Details) - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01 $ in Millions	Dec. 31, 2018 USD ($)
Remaining Performance Obligations
Remaining performance obligations	$ 1,170
Expected percentage of remaining performance obligations	85.00%
Expected timing of performance obligations	12 months